1. Field of Invention
The present invention relates to semiconductor processing methods and apparatuses that use one or more photonic beams having an initially nonuniform intensity profile to generate an image, which, in turn, is scanned across a surface of a semiconductor substrate. In particular, the invention relates to such methods and apparatuses in which the image exhibits a uniform intensity profile over a useful portion thereof such that energy utilization of the beam is increased.
2. Description of Background Art
Fabrication of semiconductor-based microelectronic devices such as processors, memories and other integrated circuits (ICs) often involves subjecting a semiconductor substrate to numerous processes, such as photoresist coating, photolithographic exposure, photoresist development, etching, polishing, and heating or “thermal processing”. In certain applications, thermal processing is performed to activate dopant atoms implanted in junction regions (e.g., source and drain regions) of the substrate. For example, the source/drain parts of transistors may be formed by exposing regions of a silicon wafer to electrostatically accelerated dopants containing either boron, phosphorous or arsenic atoms. After implantation, the dopants are largely interstitial, do not form part of the silicon crystal lattice, and are electrically inactive. Activation of these dopants may be achieved by annealing the substrate.
Annealing may involve heating the entire substrate to a particular processing temperature for a period of time sufficient for the crystal lattice to incorporate the impurity atoms in its structure. The required time period depends on the processing temperature. Particularly during an extended time period, the dopants tend to diffuse throughout the lattice. As a result, the dopant distribution profile may change from an ideal box shape to a profile having a shallow exponential fall-off.
By employing higher annealing temperatures and shorter annealing times it is possible to reduce dopant diffusion and to retain the dopant distribution profile achieved after implant. For example, thermal processing (TP) encompasses certain techniques for annealing source/drain regions formed in silicon wafers as part of the process for fabricating semiconductor devices such as integrated circuits (ICs). An objective of rapid thermal processing (RTP) is to produce shallow doped regions with very high conductivity by rapidly heating the wafer to temperatures near the semiconductor melting point to Incorporate dopants at substitutional lattice sites, and then rapidly cool the wafer to “freeze” the dopants in place. RTP is particularly useful in the context of semiconductor-based microelectronic devices with decreased feature sizes, because it tends to produce low-resistivity doped regions, which translates into faster ICs. It also results in an abrupt change in dopant atom concentration with depth as defined by the implant process, since thermal diffusion plays only a very minor role in the rearrangement of the impurity atoms in the lattice structure.
Laser-based technologies have been employed to carry out TP on time scales much shorter than those employed by conventional RTP systems. Exemplary terminology used to describe laser based TP techniques include laser thermal processing (LTP), laser thermal annealing (LTA), and laser spike annealing (LSA). In some instances, these terms can be used interchangeably. In any case, these techniques typically involve forming a laser beam into a long, thin image, which in turn is scanned across a surface to be heated, e.g., an upper surface of a semiconductor wafer. For example, a 0.1-mm wide beam may be raster scanned over a semiconductor wafer surface at 100 mm/s to produce a 1-millisecond dwell time for the heating cycle. A typical maximum temperature during this heating cycle might be 1350° C. Within the dwell time needed to bring the wafer surface up to the maximum temperature, a layer only about 100 to about 200 micrometers below the surface region is heated. Consequently, the bulk of the millimeter thick wafer serves to cool the surface almost as quickly as it was heated once the laser beam is past. Additional information regarding laser-based processing apparatuses and methods can be found in U.S. Pat. No. 6,747,245 and U.S. Patent Application Publication Nos. 2004/0188396, 2004/0173585, 2005/0067384, and 2005/0103998 each to Talwar et al.
LTP may employ either pulsed or continuous radiation. For example, conventional LTP may use a continuous, high-power, CO2 laser beam, which is raster scanned over the wafer surface such that all regions of the surface are exposed to at least one pass of the heating beam. The wavelength of the CO2 laser, λ, is 10.6 μm in the infrared region. This wavelength, large relative to the typical dimensions of wafer features, can be uniformly absorbed as the beam scans across a patterned silicon wafer resulting in each point on the wafer being subject to very nearly the same maximum temperature.
Similarly, a continuous radiation source in the form of laser diodes may be used in combination with a continuous scanning system. Such laser diodes are described in U.S. Pat. No. 6,531,681, entitled “Apparatus Having Line Source of Radiant Energy for Exposing a Substrate”, which issued on Mar. 11, 2003 and is assigned to the same assignee as this application. Laser diode bar arrays can be obtained with output powers in the 100 W/cm range and can be imaged to produce line images about a micrometer wide. They are also very efficient at converting electricity into radiation. Further, because there are many diodes in a bar each operating at a slightly different wavelength, they can be imaged to form a uniform line image.
An alternate method of annealing employs a pulsed laser to illuminate an extended area and a step-and-repeat system. In this case, a more uniform temperature distribution can be obtained with a longer radiation pulse (dwell time) since the depth of heating is greater and there is more time available during the pulse interval for lateral heat conduction to equalize temperatures across the circuit. However, longer dwell times require more pulse energy. Pulse lengths with periods longer than a microsecond and covering circuit areas of 5 cm2 or more are not typically feasible because the energy per pulse becomes too high. While technically possible, the laser and associated power supply needed to provide such a high-energy pulse will likely be impractically big and expensive.
In general, illumination uniformity (both macro- and micro-uniformity) over the useable portion of the exposure image is a highly desirable trait. This ensures that the corresponding heating of the substrate is equally uniform. Similarly, the energy delivered in each beam pulse should be stable so that all exposed regions are successively heated to a uniform temperature. In such a system, the size of the uniformly illuminated area may be adjusted to contain an integer number of circuits. In addition, the illumination fall-off beyond the edge of the usable portion of the exposure image is preferably sufficiently sharp, so that there is no appreciable exposure of adjacent circuits on the substrate. Defining the edges of the illumination pattern with a resolution of about 50 microns is usually sufficient since the scribe lines separating adjacent circuits are typically at least that wide.
Certain “nonmelt” LTP techniques involve shaping the beam from a continuous CO2 laser to form an image of about 0.12 mm wide and over 10 mm long, which is incident on the wafer at Brewster's angle (˜75° incidence). It is desirable to have the beam incidence angle contained in a plane normal to the wafer surface and aligned with the length of the image. The beam is scanned over the substrate in a direction perpendicular to its long direction. Even if a beam intensity uniformity of 1% can be achieved over the length of the image, this results in a corresponding 10° C. or 14° C. temperature difference along the beam depending on whether the background substrate temperature starts at 400° C. or room temperature, respectively.
In some instances, then, e.g., for semiconductor annealing applications, a highly uniform intensity along the length of the beam, e.g., to about 1%, may be desired. In this case if a beam having a Gaussian intensity profile is employed, only the central portion of the beam that exhibits a substantially uniform intensity, e.g., to about 1% or less, may be used. This useful portion contains only about 11% of the total energy in the beam. The remaining energy may be wasted or may contribute to undesirable heating of adjacent regions.
Thus, opportunities exist in the art to improve the performance of TP techniques to overcome the drawbacks associated with known LTP techniques that involve the use of one or more radiation beams having a nonuniform intensity profile. In addition, there exist opportunities in the art to meet the need for LTP technologies that exhibit improved energy utilization.